Carbon nanotubes (CNT) are produced by arc discharge between two carbon materials, and are graphene sheets, in which carbon atoms are regularly arranged in a hexagonal form, rolled up into cylinders. A single-layer cylinder of a graphene sheet is referred to as a single-walled carbon nanotube (SWCNT), and its diameter is between 1 nm and several nanometers. A cylinder in which a plurality of graphene sheets are concentrically nested is referred to as a multi-walled carbon nanotube (MWCNT), and its diameter is between several nanometers and tens of nanometers. The single-walled carbon nanotube is conventionally synthesized by arc discharge using a carbon electrode containing a metal catalyst, or an anode in which a metal catalyst is embedded. The above-mentioned carbon material refers to an amorphous or graphitic conductive material principally containing carbon (the same applies to the following description).
In any case, various processes for synthesizing carbon nanotubes (CNT) by arc discharge between two carbon materials have been proposed. For example, a process has been proposed in which carbon nanotubes are produced by direct-current carbon arc discharge in an airtight chamber filled with helium or argon at an internal pressure of 200 Torr or more (for example, in Japanese Unexamined Patent Application Publication No. 6-280116).
Another process has also been proposed in which carbon nanotubes having uniform length and diameter distributions are produced by a direct-current arc discharge between carbon bar discharge electrodes in an airtight chamber filled with helium, with the internal temperature controlled in the range of 1,000 to 4,000° C. (for example, Japanese Unexamined Patent Application Publication No. 6-157016).
In view of: (a) achieving continuous collection of deposit on the cathode, which is generally collected in a batch process; (b) preventing the arc from becoming unstable as the deposit grows; (c) preventing decrease in yield resulting from long-time arc exposure of the deposit on the cathode; and (d) producing carbon nanotubes over a large area of the surface of the cathode, a process has been proposed in which carbon nanotubes are produced by arc discharge between opposing electrodes which are horizontally disposed in an airtight chamber filled with an inert gas, and which are relatively turned or reciprocated, continuously or intermittently (for example, Japanese Unexamined Patent Application Publication No. 7-216660).
Another process has also been proposed in which graphite fibers are produced on an disk-shaped cathode by arc discharge in an atmosphere containing at least one gas selected from among air, oxygen, and nitrogen, with the cathode continuously or intermittently rotated (for example, Japanese Unexamined Patent Application Publication No. 2002-88592).
A technique for increasing the purity and yield of carbon nanotubes has also been proposed in which opposing carbon anode and cathode disposed in an airtight chamber are heated by a surrounding heater before arc discharge between the electrodes (for example, Japanese Unexamined Patent Application Publication No. 2000-203820.
A technique efficiently producing carbon nanotubes having a uniform length and quality also has been proposed in which the tip of the anode constituted by a carbon electrode disposed in an airtight chamber is heated by heating means before arc discharge (for example, Japanese Unexamined Patent Application Publication No. 2000-344505).
These processes cited above, however, have the following technical disadvantages.
Specifically, although carbon nanotubes are produced in a substance constituted of carbon atoms deposited on the carbon cathode of the arc discharge portion, or in part of soot which flied off and deposited in the vicinity of the arc, the above-cited known processes for producing carbon nanotubes cannot prevent the presence of substances other than carbon nanotubes, such as graphite and amorphous carbon, in the product, and the carbon nanotube content in the product is low, accordingly.
In general arc discharge, its cathode spot occurs selectively in an area having a high electron emission property. The electron emission from the cathode spot gradually decreases, and the cathode spot moves to another area having a higher electron emission property. Thus, an arc is generally discharged while the cathode spot is moving irregularly and vigorously. In some cases, the cathode spot largely deviates from the area opposing the anode to increase load beyond the voltage limit of the power supply, and consequently arc is extinguished. In the arc discharge in which the cathode spot moves irregularly and vigorously, chemical factors, such as temperature and carbon vapor density, in an area of the cathode largely vary with time. Accordingly, synthesis conditions are varied, so that carbon nanotubes may be easily produced in a certain time period, but may be hardly produced or become liable to decompose in another time period. Consequently, carbon nanotubes containing a large amount of impurities are produced over the areas where the cathode spot occurred. The decomposition of carbon nanotubes herein means a phenomenon that their carbons structurally change into a form of graphite or amorphous carbon when they are, in a certain temperature range, more stable in a form of graphite or amorphous carbon than in a form of carbon nanotube, or that a group (cluster) of the carbon atoms constituting the produced carbon nanotubes are released to decompose the carbon nanotubes at a fairly high temperature, although it cannot be said definitively because the mechanism of production of carbon nanotubes has not yet been clear completely. Since production process of carbon nanotubes itself is performed at a high temperature, the cluster is probably released during the production. At temperatures optimum for production of carbon nanotubes, however, carbon nanotubes are produced probably because the production speed of carbon nanotubes are higher than the decomposition speed (cluster release speed).
In the known processes, therefore, equipment for arc discharge is disposed in an airtight chamber in which the constituent and pressure of the atmospheric gas and the temperature around the electrodes are appropriately selected and controlled, as described above, in order to stabilize the arc and increase the production rate of carbon nanotubes.
Unfortunately, it is difficult to completely fix the cathode spot only by adjusting the constituent and pressure of the atmospheric gas and the internal temperature or temperature around the electrodes in the airtight chamber. Accordingly, the resulting carbon nanotubes are collected only in deposit on the cathode or soot which is a mixture of a large amount of impurities and the carbon nanotubes. Consequently, the yield of the carbon nanotubes is reduced, and a complicated purification process is required to increase the purity of the carbon nanotubes. Thus, production cost is increased. Furthermore, the size of equipment is increased to raise equipment costs and make difficult mass production of carbon nanotubes by arc discharge.
In one of the above-described processes, the electrodes are relatively moved in order to produce carbon nanotubes continuously with a high density in a high yield. However, this process is still intended to continuously collect the deposit on the cathode, containing a large amount of impurities. High-density carbon nanotubes can be produced in some cases by increasing relative speed. However, the resulting carbon nanotubes have a thickness of about 100 μm, and are difficult to collect even with a blade-like scraper or the like. In the electrode-moving process, the electrodes are repeatedly moved in the same region, and consequently the temperature of the cathode gradually increases, so that the temperature history at the arc generation point is changed. It is therefore impossible to constantly produce high-purity carbon nanotubes in a high yield.
The carbon nanotube is expected to be used for cathode materials of fluorescent display tubes, field emission displays (FED), and the like and electron microscope probes, as field emission electron sources because of their nanofibrous structure and high crystallinity. However, since the known processes produce carbon nanotubes in a powder or aggregate form containing a large amount of impurities, the purification process is troublesome, and handling or processing is also troublesome.
When carbon nanotubes are compressed, or immersed in a liquid and then dried, they are aggregated by van der Waals force. Accordingly, the carbon nanotubes are formed into an aggregate or a thick bunch through, for example, grinding in a purification step or treatment with an acid solution, and consequently the nanofibrous structure of the carbon nanotubes is lost. If carbon nanotubes are formed into an aggregate or a thick bundle, so that they cannot have a nanofibrous structure, the capability to serve as a field emission electron source is considerably degraded.
Carbon nanotubes produced by arc discharge generally have higher crystallinity and higher quality than carbon nanotubes produced by thermal decomposition. However, it is impossible to directly synthesize a carbon nanotube film on a substrate of silicon or the like by an arc process because arc temperature is high. For such a film, it is therefore necessary to adopt thermal decomposition, or to thinly spread carbon nanotube powder produced by arc discharge and bond it in some way. Unfortunately, thermal decomposition cannot provide high-quality carbon nanotubes. Also, if carbon nanotube powder produced by a known arc discharge process is used, the powder is not uniformly distributed on the substrate and unevenness occurs, disadvantageously.
In order to uniformly put powder or aggregate of carbon nanotubes on a substrate or an electrode to use as an electron source for field emission, a method is applied in which the carbon nanotubes are dispersed in a conductive paste (for example, silver paste) and applied onto the substrate or electrode, followed by drying and firing, and the resulting surface is then polished or treated with laser light or plasma so as to expose the carbon nanotubes at the surface. It is however difficult that this method provides stable quality suitable as a field emission electron source, and the method makes the production process complicated and sophisticated, thus increasing production costs.